300D Speedometer problem!
 

I have a 1983 300D and my speedometer is gone wonky! My odometer is not working and my speedo is reading incorrectly. When it reads 30 MPH, I'm actually going 24 MPH! there seems to be a 2 MPH discrepancy for every 10 MPH of speed and this increases exponentially! What can i do about this? I know I'm going to need a speedo. Is it possible that my 300D was made for a 85 MPH speedo instead of a 120 MPH speedo? Thanks. I know that working for the Chrysler Corp, they would have a gear reduction mechanism to correct this error, but how about Benz? I'm getting conflicting responses from shops around here!:confused:

my 240D with the 85 MPH speedo has the same problem, i found this out using my GPS, which so conviniantly could track how fast you were going via satalite. I concluded that it is 6MPH OFF!! all the time. i just keep up with traffic flow and im usually fine

My problem is 2 MPH exponentially! It reads 2 MPH off at 10 MPH, 4 MPH off at 20 MPH, 6 MPH off at 30 MPH and so on. It's reading faster than I'm really going! This sucks and I'd like to fix it!:mad:

You odometer can probably be fixed by following Diesel Giant's Odometer Repair.

Are you sure you have the correct tire size, if they are the wrong size they will throw you speedo off.

my '83 240D was off by 5mph at 60mph. So in need of new tires, I put on 185-75- 14. these are a little taller tires which put me at 59mph when the speedo read 60. Works for me.

Yes, my tyres are the P195/70R14 size! Did any Mercedes Benz 300D's come from the factory with the 85 MPH speedometer?:confused: The previous owner did a transmission swap and maybe the speedo gear in the tranny is different?

Your 1983 300d should come with 120 MPH speedometer. Look on the back of the speedo for a white tag with 3 sets of #s with some of these / / flanking the middle number this is the ratio. (not sure what all this means but I learned this from mbtechman, he states the numbers have to match for it to work) My '83s number is 99/4/52 and my '84s numbers are 99/4/56 they don't jive when I test it. My guess you have a later model speedo (you can check for the date stamped on the back or your transmission is not set up right after the swap.

The 1982 had a factory 85 mph speedo.

If the tranny swap made by the previous owner came from a 1982, would this be the older style 85 MPH gear then? How would I go about changing this? Can i get a speedo gear from a newer tranny?:o

I am not the best person to tell you the sure way to do this. Also I am not sure what size speedometer gearing would be in your replacement transmission. But if you were to go the route of changing the gear in the transmission, you would have to take the whole transmission tail off to get to the gears. Most people don't do that and end up taking their speedometer to a speedo shop to have it calibrated. They usually calibrate the speedometer itself and nothing else.

I don't think that's your problem. The cable runs at the same speed, it's the speedo itself that is different. I have at 125 mph speedo (from a 83 or 84 300D) in my 82 300D and it works correctly. They all have the same 3.07 final gear ratio. The 85 300D has a 2.88 final ratio, so the speedo is calibrated differently.

That makes sense then because if the tranny swap was from a 1985 300D then my speedo would be reading slower than I'm actually going because of the 2.88 gear ratio! Thanks! I do need to have mine repaired too so this helps.;)

My 82 wagon has the 120mph speedo, so not all of them came with a 85mph speedo

You mean linearly. It increases 2 mph for every 10 mph you're going.

If it were exponentially, you'd see 12 mph when you're going 10, 24 mph when you're going 20, 38 mph when you're going 30, 56 mph when you're going 40, 82 mph when you're going 50, 124 mph when you're going 60, and 198 mph when you're going 70.

Gotta love math! :D